How to buy a UPS

A UPS (uninterruptible power supply) keeps your equipment running for a short time during a power cut and protects it from surges. These six steps help you pick the right one.

1. Add up your load in watts

Check the label on each device you want to protect. A typical desktop PC uses 150 to 400 W, a monitor 20 to 60 W, and a router 10 to 20 W. Add them up.

2. Leave headroom

Add 20 to 25 percent to your total. A UPS that runs near its limit has shorter runtime and ages faster.

3. Convert to VA

UPS capacity is listed in VA and in watts. Check the watt rating on the box, because it is the number that counts. If only VA is shown, assume watts are 60 to 90 percent of VA, depending on the model.

4. Choose the type

• Standby (offline): cheapest, fine for basic home use.
• Line-interactive: corrects voltage swings without using the battery. The best choice for most homes and small offices.
• Online (double conversion): constant clean power for servers and critical gear.

5. Check the waveform and runtime

Choose a pure sine wave model for gaming PCs and servers with active PFC power supplies. Look at the manufacturer's runtime chart to see how many minutes you get at your load.

6. Compare features and warranty

• Number of battery-backed outlets versus surge-only outlets.
• USB or network port for safe shutdown software.
• Tower or rack-mount form.
• Warranty length and the price of a replacement battery.

Most home users do well with a line-interactive UPS rated 600 to 1500 VA. Plan to replace the battery every 3 to 5 years.